Aluminum nitride exhibits a higher surface wave propagation speed than piezoelectric materials such as ZnO and PZT and thus has been noted as a material for a high frequency surface acoustic wave element. For this purpose, a single crystal aluminum nitride is preferably used. However, since a large-sized single crystal aluminum nitride cannot be produced by the techniques that are currently available, it is a common practice to form, on a substrate of a different kind material, a single crystal aluminum nitride film or a polycrystalline aluminum nitride film oriented in a predetermined direction. The orientation of the single crystal aluminum nitride film and that of the polycrystalline aluminum nitride film are normally such that the C-axis of the film is perpendicular to the substrate. As the substrate of a different kind material on which the aluminum nitride film is formed, glass, which is inexpensive, and sapphire, on which a single crystal aluminum nitride can be formed, are generally used.
As a substrate of a different kind material which can propagate wave at a high speed and exhibits a high electro-mechanical coupling coefficient when aluminum nitride is formed thereon, diamond capable of propagating wave at the highest speed among all substances can be used, as disclosed in JP-A-2-20910 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). This construction, in which an aluminum nitride film is formed on diamond, concerns a surface acoustic wave device comprising an aluminum nitride film formed on a crystalline diamond film formed on a single crystal semiconductor substrate. The results of analysis of the surface acoustic wave device having a two-layer construction comprising a (100) plane [100] direction crystalline diamond film and a (001) plane aluminum nitride (hereinafter sometimes abbreviated as "AlN") film are disclosed in the drawings in JP-A-2-20910. In this known example, an amorphous silicon dioxide film is formed on the (100) plane of a diamond single crystal film or a diamond polycrystalline film formed on the (100) plane of a single crystal semiconductor substrate. An aluminum nitride film is formed on the silicon dioxide film to obtain an excellent C-axis oriented film.
JP-A-5-890888 proposes a process for producing a C-axis oriented aluminum nitride film on a (100) direction oriented diamond polycrystalline film. It proposes that a seed crystal, from which a (100) direction oriented crystalline diamond film is formed on a silicon substrate, be formed on the silicon substrate.
JP-A-64-42813 proposes a construction in which a single crystal aluminum nitride is formed on a single crystal diamond, which acts as a substrate having an excellent heat resistance and environmental resistance.
In the above cited JP-A-2-20910, the results of analysis of KH dependence of the phase velocity of the surface acoustic wave in the two-layer construction comprising a (100) plane [100] direction crystalline diamond film and a (001) plane AlN film are set forth in FIG. 11. However, this analysis is of an ideal condition, but there is no disclosure of the actual degree of orientation of the (100) plane [100] direction crystalline diamond film and the (001) plane AlN film.
The technique of the above cited JP-A-2-20910 involves a problem in the formation of an aluminum nitride film on an amorphous silicon dioxide film formed on a diamond film. The aluminum nitride film has an insufficient orientation since it is formed on an amorphous film. Further, since silicon dioxide, which propagates wave at a lower speed than diamond and aluminum nitride, is present interposed between the diamond film and the aluminum nitride film, the inherent superiority of the combination of high wave velocity substances, i.e., diamond and aluminum nitride, cannot be sufficiently utilized.
The technique of the above cited JP-A-5-090888 is not clear whether the formation of an aluminum nitride film on a (100) direction oriented diamond polycrystalline film can provide a good C-axis orientation. The above cited JP-A-64-42813 proposes the (111) plane of a single crystal diamond as the crystal surface on which aluminum nitride grows. However, there is no disclosure of the plane direction of aluminum nitride which grows on the (111) plane of the single crystal diamond.